This invention relates to novel block copolymers for use in pressure sensitive adhesives. More particularly, it relates to branched polystyrene-polyisoprene-polybutadiene block copolymer compositions and pressure sensitive adhesives containing such compositions.
It is known that radial block copolymers can be obtained by anionic copolymerization of a conjugated diene compound and an alkenyl arene compound by using an organic alkali metal initiator. Block copolymers have been produced which comprise primarily those having a general structure EQU (A--B).sub.n X
wherein the polymer blocks A comprise thermoplastic polymer blocks of alkenyl arenes such as polystyrene, block B is a polymer block of a conjugated diene such as polyisoprene, and n is the number of branches. When the content of the alkenyl arene is small, i.e. 5-30% the produced block copolymer is a so-called thermoplastic rubber. In such a rubber, the A blocks are thermodynamically incompatible with the B blocks. Such block copolymers have been used to make pressure sensitive adhesives for a wide variety of uses including tapes, labels and product assembly applications.
Radial block copolymer commercially available under the trade name QUINTAC.TM. 3450 from Nippon Zeon are predominately of the (S--I).sub.3 X type. The coupling efficiency of the resulting copolymer is approximately 70%. Recently, in U.S. Pat. No. 5,194,500, a three-armed (S--I).sub.3 X block copolymer was described of (polystyrene equivalent molecular weight) between 180,000 and 250,000. The coupling efficiency of the copolymer was defined to be greater than 85%. The patentees stated that a direct correlation exists between increased coupling efficiency and holding power in shear.
Processing expenses as well as the need for higher cohesive strength dictate the need for more efficacious radial block copolymers. Optimum balance between high holding power and low melt viscosity has not to date been attained by the block copolymers of the prior art.
A need exists for radial block copolymers of higher holding power and shear adhesion failure temperature (SAFT) than the polymers of the prior art. An increase in these indices would permit the use of higher levels of tackifying resins and plasticizing oils in adhesive formulations from those levels presently employed. Tackifying resins and plasticizing oils are generally the least expensive components of pressure sensitive adhesive formulations.